Plasmas are employed in materials manufacturing for a diverse range of processes, including surface activation, etching, cleaning, sterilization, decontamination, and thin-film deposition. For example, U.S. Pat. No. 8,029,105 describes treating a printer component with a plasma. Plasmas operate either at low pressure, for example <5 Torr, or at atmospheric pressure, for example about 760 Torr (see for example, Lieberman and Lichtenberg, “Principles of Plasma Discharges and Materials Processing,” John Wiley & Sons, Inc., New York, 1994; Chen, “Introduction to Plasma Physics and Controlled Fusion,” Plenum Press, New York, 1984; and Roth, “Industrial Plasma Engineering: Vol. I, Principles” Institute of Physics Publishing, Philadelphia, Pa., 1995). The low-pressure devices are operated in a batch mode, and find wide application in semiconductor fabrication. By contrast, atmospheric-pressure plasmas can be operated in a continuous mode on an assembly line, and are more common in web conversion, automotive, aerospace, and specialty materials industries.
Low-temperature, atmospheric-pressure plasmas are weakly ionized discharges for which only a small fraction of the gas molecules become ionized (see Schutze, et al., “The Atmospheric-Pressure Plasma Jet: A Review and Comparison to other Plasma Sources,” IEEE Transactions in Plasma Science, vol. 26, page 1685 (1998)). These systems are not at equilibrium, because the temperature of the free electrons is several orders of magnitude higher than the temperature of the neutral species. Several types of non-equilibrium, atmospheric-pressure plasmas have been developed over the years. These include coronas, dielectric barrier discharges, micro-hollow cathode discharges, and radio frequency (RF) powered capacitive discharges. Nehra, Kumar, and Dwivedi divide atmospheric non-thermal plasma sources into four categories: corona discharges, dielectric barrier discharges (DBD), atmospheric-pressure plasma jets (APPJ), and micro-hollow cathode discharges (MHCD) (V. Nehra, A. Kumar, and H. K. Dwivedi in International Journal of Engineering 2008 Vol (2) issue (1) p. 53-68 CSC open access journal).
A corona is an electrical discharge in which ionization takes place in a region of high electric field. The most common type of corona is the point-to-plane design, where one of the electrodes is a narrow wire or a metal tip and the other electrode is planar (see Goldman and Goldman, “Corona Discharges” Gaseous Electronics, vol. 1, (Eds: Hirsh and Oakam), Academic Press, New York, 1978). Power, at frequencies ranging from 50 Hz to 20 kHz, is supplied to the pointed electrode, creating a high electric field that promotes breakdown of the gas in the vicinity of the electrode. A localized, luminous discharge is created around the tip of the powered electrode. Since the plasma density falls rapidly away from the sharp tip, one must pass the substrate very close to the electrode for the substrate to be processed at a suitable rate. Therefore, this device is for the most part restricted to treating plastic film or fabric that is continuously passed through the plasma in a roll-to-roll format.
Dielectric barrier discharges, also known as “silent” discharges, operate with two metal electrodes, in which at least one is coated with a dielectric material. The metal electrodes are separated by a uniform gap, and are powered by direct current (DC) or alternating current (AC) at frequencies up to 50 kHz. In most cases, dielectric barrier discharges operate in a “filamentary” or “micro-discharge” mode, where the plasma exhibits short-lived micro arcs that are randomly distributed in space and time (see Eliasson and Kogelschatz, IEEE Transactions in Plasma Science, vol. 19, page 1063, 1991). A uniform, diffuse glow mode can be obtained in a dielectric barrier discharge if an inert gas such as helium, argon, or nitrogen is used as a diluent. The electron density in these plasmas varies over a wide range depending on whether the gas is sampled inside or outside a streamer. Nevertheless, the average electron density is low, about 109 cm−3, which means that, as with a corona, one must insert the substrate into the plasma between the electrodes to obtain a suitable surface treatment rate. Dielectric barrier discharges are primarily employed in the surface activation of plastic films and paper surfaces.
Micro-hollow-cathode discharges are direct-current glow discharges sustained between two parallel metal electrodes with a center opening of 0.1 mm in diameter in either the cathode, or the cathode and the anode (see Stark and Schoenbach, Applied Physics Letters, vol. 74, page 3770, 1999; and Bardos and Barankova, Surface Coating Technologies, vol. 133-134, page 522, 2000). The electrodes are separated by a gap of 0.2 to 0.4 mm, which is often filled with a dielectric material. Gas, such as argon, xenon, or air is passed through the hole where it is ionized by application of DC, or in a few cases, RF power. The plasma density is highest inside the hole at 1014 cm−3, and quickly decreases in density outside of this region. Hollow-cathode discharges are mostly used as light sources and processing materials with these devices has been limited.
Capacitive discharges are also called capacitively coupled atmospheric-pressure plasmas and this type of atmospheric discharge also relies on small inter-electrode gaps to reduce the operating voltage required to sustain the plasma; however, capacitively coupled plasmas are driven by AC voltage and the plasma characteristics depend on the operating frequency. Below about 500 kHz atmospheric plasmas generally will be extinguished, meaning become non-conductive, between voltage cycles, partly due to the small mean free path of gaseous species at atmospheric pressure and partly due to the accelerated recombination kinetics occurring at atmospheric pressure. Above about 500 kHz atmospheric-pressure plasmas remain conducting through the entire AC voltage cycle. These observations have driven the considerable interest in discharges at atmospheric pressure operating at frequencies higher than about 500 kHz.
A non-equilibrium, atmospheric-pressure discharge can be produced by flowing gas between two closely spaced metal electrodes that are driven with high-frequency power (see Koinuma et al., U.S. Pat. No. 5,198,724; Li et al., U.S. Pat. Nos. 5,977,715 and 6,730,238; and Selwyn, U.S. Pat. No. 5,961,772). These plasmas have been used to process materials placed a short distance downstream of the electrodes.
In U.S. Pat. App. Pub. No. 2002/0129902 A1 entitled “Low-Temperature Compatible Wide-Pressure-Range Plasma Flow Device,” dated Sep. 17, 2002, Babayan and Hicks describe an apparatus that comprises a housing with two perforated metal electrodes, Gas flows through the electrodes and is partially ionized by applying radio frequency power to one of the electrodes at 13.56 MHz. Radicals produced in the plasma flow out of the device and can be used to treat substrates placed a short distance downstream. It was observed that the etch rate of photoresist with an oxygen and helium plasma at 760 Torr was between 0.4 and 1.5 microns per minute over a circular area 30 mm in diameter. U.S. Pat. No. 8,328,982 by Babayan and Hicks entitled “Low-Temperature, converging reactive gas source and method of use” describes the construction of atmospheric-pressure plasma sources with shaped gas chambers that produce an inward converging flow of gas between electrodes in the plasma generating region towards the gas chamber outlet. U.S. Pat. No. 8,328,982 also discloses a type of precursor distributor for feeding one or more precursors chemicals into the gas flow from the plasma to introduce new and unique reactivity to the exiting gas flow.
Atmospheric-pressure plasma sources are remote plasma sources which means that the gas-phase reactive species generated in the plasma zone create a reactive fluid flow that exits the plasma source as a fluid flow and are transported by the fluid flow to the substrate surface. The fluid flow that transports the plasma-excited reactive gas is usually called a jet. The jet is typically directed to impinge on the surface of an opposing workpiece or substrate to treat the surface with the plasma-excited reactive gas.
There is extensive scientific literature in the art of heat transfer concerning jet impingement of compressible and incompressible fluids on a surface because of numerous industrial processes using impinging gas jets for heat transfer. The vast majority of industrial applications involve the use of jet impingement for cooling and more recently micro-jet impingement has been investigated for microelectronics cooling applications. Jet impingement is cost effective and extremely efficient and provides a simple method for achieving high heat transfer coefficients. Heat transfer studies of impinging jets focus on spatial characterization of the Nusselt number for the flow. The Nusselt number is the ratio of convective to conductive heat transfer for the flow at any point and is a measure of the effectiveness of heat transfer across a boundary. The Nusselt number by itself does not describe gas entrainment or the decay of plasma chemical species. The available information in the heat transfer art yields therefore does not anticipate how jet impingement configuration will affect a plasma chemical process.
The art of aeronautics has extensive literature concerning jet propagation and impingement. Fluid flow studies of impinging jets are of some value because these studies attempt to characterize the velocities fields of the flow. Donaldson and Snedeker commented that those skilled in the art of fluid mechanics of impinging jets understand that “Each free jet in its own particular laboratory has its own special idiosyncrasies” (C. Du P. Donaldson and R. S. Snedeker in J. Fluid Mech. (1971) 45(2), pp 281-319 quote taken from page 281 Introduction section). From this comment made by experts in the art of jet impingement fluid mechanics, it is clear that, although some general comments about a particular jet impingement configuration can be made, specific characteristics of a jet configuration for an application are not obvious and a particular jet impingement configuration cannot be predicted and must be empirically tested for efficacy.
Some of the first analytical work concerning outward radial flow of impinging gas jets was published in 1956 by M. B. Glauert (M. B. Glauert, “The Wall Jet” J. Fluid Mech. 1, 625, (1956)). Glauert focused on analytically describing the behavior of a free unimpeded subsonic jet of gas when the jet strikes a surface at right angles then radially spreads outward over it. This is known as free jet impingement. As the jet impinges on a surface a stagnation zone is formed underneath the jet and the pressure at the stagnation zone is essentially that of the jet itself. The jet spreads and flows over the stagnation zone and begins to flow over the surrounding surface. The velocity of this spreading fluid is somewhat less than the jet itself. Glauert called the radially spreading fluid a “wall jet” and attempted to analyze the behavior of the fluid flow, focusing specifically on the velocity profile of the fluid normal to the wall as it propagates across the wall surface. As the fluid flows along the wall surface, boundary and shear layer interactions occur that eventually result in the detachment of the wall jet from the surface. The wall jet detachment is characterized by an increase in turbulence of the flow around the detachment point as the wall jet fluid mixes with the fluid above it. Glauert's analysis of free jet impingement and wall jet behavior is often thought of as a starting point for understanding the mass and heat transfer that result from free and confined jet impingement.
Garimella (S. V. Garimella, Annual Rev of Heat Transfer vol. 11, (2000), Chapter 7 on pp. 413-494, “Heat Transfer and Flow Fields in Confined Jet Impingement”) characterized the heat transfer properties of confined jet impingement in various configurations. For example, Fitzgerald and Garimella (J. A. Fitzgerald and S. V. Garimella, Int. J. Heat Mass Transfer, 41 (8-9) (1998), pp. 1025-1034, “A study of the flow field of a confined and submerged impinging jet”) used laser Doppler velocimetry of confined submerged jets to examine the turbulent toroidal recirculation patterns found in the transition zone that are a unique characteristic of incompressible confined submerged jets.
U.S. Pat. No. 8,643,173 describes the use of confined jet impingement in a cooling apparatus with surface enhancement features that are used to induce bubble nucleation of a cooling fluid for enhanced heat transfer integrated into power electronics modules.
Lytle and Webb (D. Lytle and B. W. Webb, Int. J. Heat Mass Transfer 37(12) (1994) 1687-1697, “Air jet impingement heat transfer at low nozzle-plate spacings”) studied the heat transfer of free jet impingement using jets of air at low nozzle-plate spacings and used Doppler laser velocimetry to establish the characteristics of confined outward radial flow when the gas between the two flow confining surfaces is small relative to the nozzle diameter. The Reynolds number (Re) of the jet was between 3600 and 27600 and the fluid behavior was considered incompressible because the gas velocities used in the study are subsonic and significantly below Mach 1 (the speed of sound). Sonic and supersonic gas velocities found in underexpanded jets suggest that the fluid be treated as compressible to take into account the formation of shock.
Lytle and Webb studied the behavior of free jet impingement with respect to the dimensionless parameter z/d. The dimensionless parameter z/d is the ratio of the spacing between the confinement surfaces 101 to the diameter of the jet emitting nozzle 112. The geometry used by Lytle and Webb was identical to FIG. 1A. Lytle and Webb made several important observations:
1) Gas entrainment occurs due to accelerated flow outward at the nozzle edge that is especially pronounced at z/d<0.25.
2) Virtually no gas entrainment was observed at z/d>0.5 for Re<15000.
3) Sub-atmospheric-pressure regions are formed just outside the nozzle edge at low nozzle plate spacing (z/d=0.1) with high Re numbers (13000). The radial dependence of the static pressure of the radial flow shows that as z/d is decreased the radial dependence of the pressure drop becomes more pronounced. The investigators attribute this to a vena contracts effect where the fluid emerging from the jet nozzle accelerates as it propagates into the surrounding free space and the local acceleration of the jet causes the jet to temporarily decrease in volume because the fluid pressure drops during the acceleration. There is also a further local acceleration of the fluid during outward radial expansion after impingement. The sub-atmospheric-pressure regions formed at high Reynolds numbers are associated with gas entrainment into the core of the jet.
4) Increased heat transfer efficiency at nozzle to plate spacings that are less than 1 nozzle diameter are associated with localized flow acceleration near the nozzle edge and with increased turbulence at radial positions outside the nozzle edge that are likely associated with detachment of the wall jet from the impingement surface.
Lytle and Webb's observations of free jet impingement fluid flow suggest that free jet impingement is disadvantaged for delivery of reactive species in a fluid flow to an object surface—primarily because of gas entrainment. Decreasing the nozzle to object surface distance should improve transport of reactive species to the surface but Lytle and Webb's study show that the same conditions lead to increased gas entrainment into the jet. Gas entrainment into a jet containing reactive species is highly undesirable because secondary reactions can decrease the concentration of reactive species in the fluid flow. Although no gas entrainment is observed at larger z/d for low Reynolds numbers, at the lower Reynolds number (Re) the mass transport of a reactive species to the surface is slower and the distance the reactive species must traverse to the impingement surface is high. As a result, secondary reactions between reactive species in the jet can decrease the concentration of the reactive species in the fluid flow. As expected, experimental observations reported in the scientific literature show that etching and the surface modification effectiveness for free jet configurations of atmospheric-pressure plasma sources is low for large z/d and low Re. As mentioned previously, although the sub-atmospheric-pressure region observed outside the nozzle edge is indicative of enhanced mass transport of the wall jet, the rapid pressure drop associated with radial expansion of the free jet impingement is experimentally demonstrated to be turbulent and subject to gas entrainment of the surrounding air. Under these conditions, the enhanced mass transport provided by the wall jet is tempered by the enhanced secondary reactions of the reactive species with entrained gas and the latter reactions dominate the reactive gas chemistry when turbulence is present. Moreover, the improved heat transfer observed at radial positions outside the nozzle edge indicates increased mass transport to and from the surface but the mass transport enhancement is due to turbulent mixing of the shear layer above the wall jet as the wall jet detaches from the surface. As a result, the transport of reactive species to the surface is not favored. Virtually all known atmospheric-pressure plasma sources in the prior art employ a free jet impingement configuration.
Gillespie et al reported a flow field and heat transfer study of confined jet impingement of air jets using an experimental configuration that modeled the impinging jets at higher Reynolds numbers used for cooling in jet turbine engines (D. R. H. Gillespie, S. M. Guo, Z. Wang, P. T. Ireland, and S. T. Kohler, paper number 96-GT-428, International Gas Turbine and Aeroengine Congress and Exhibition, Birmingham, UK, Jun. 10-13, 1996, “A comparison of full surface local heat transfer coefficient and flow field studies beneath sharp-edged and radiused entry impinging jets”). Gillespie et al used a fixed nozzle to plate distance (z/d) for the confined jet of approximately 1.25 and investigated the pressure and flow characteristic of outward radial flow at Reynolds numbers between 16000 and 40000. Heat transfer measurements indicated the presence of a transition region at r/d=1.2 that was interpreted as being associated with turbulence occurring during the transition from the stagnation zone to the wall jet. The static pressure distribution measurements indicated a sub-atmospheric-pressure region associated with the wall jet that is formed several nozzle diameters outside the edge of the nozzle, although there was no interpretation or commentary on these characteristics of confined jet impingement.
Baydar (E. Baydar, Experimental, Thermal, and Fluid Science 19(1999) 27-33, “Confined impinging air jet at low Reynolds numbers”) investigated confined jet impingement of air jets at low Reynolds numbers. Baydar investigated confined jet impingement over a Reynolds number range of 300 to 10000 and a nozzle diameter-to-plate spacing ratios ranging from 0.5 to 4. The report documents the observation of sub-atmospheric-pressure region on the impingement plate for z/d<2 for Reynolds numbers greater than about 2700. This work established a lower limit for the conditions under which sub-atmospheric-pressure regions can be observed during confined jet impingement.
Baydar and Ozmen (E. Baydar, Y. Ozmen; Heat and Mass Transfer, February 2006, Volume 42, Issue 4, pp 338-346, “An experimental investigation on flow structures of confined and unconfined impinging air jets”) investigated the flow characteristics of both confined and unconfined air jets impinging normally onto a flat plate using a smoke-wire technique to visualize the flow behavior. The mean and turbulence velocities and surface pressures were measured for Reynolds numbers ranging from 30,000 to 50,000 and nozzle-to-plate spacings in the range of 0.2-6. They concluded that confined impingement jets always show a flow region with sub-atmospheric pressure whilst there is no evidence of the sub-atmospheric region in unconfined impinging jet in the experimental space examined.
Cavadas et al (A. S. Cavadas, F. T. Pinho, J. B. L. M. Campos, Journal of Non-Newtonian Fluid Mechanics, 169-170 (2012), 1-14) studied the impinging jet flow confined by sloping plane walls and showed that the essential features of the impinging jet are retained even in the presence of non-uniform spacing between the confining surfaces.
All known atmospheric-pressure plasma and micro-plasma sources appear to use unimpeded free flowing jets as a method to deliver the reactive species from the plasma to the surface of the object to be treated. However, such free flowing jets are subject to severe gas entrainment that often require the use of a supplemental barrier such as an inert gas curtain or a physical enclosure in order to ensure that the plasma-excited reactive gas flow has sufficient reactive species present when impinging on a surface. Furthermore, ambient gas entrainment into the plasma-excited gas flow is a serious problem for atmospheric-pressure plasma sources and known solutions, such as physical enclosures and gas curtains, are cumbersome. At present there is no other known method for addressing gas entrainment for atmospheric-pressure plasma sources. Thus, simple, realistic solutions to the gas entrainment problem and to improving the mass transport of plasma-excited reactive gases to a substrate surface at atmospheric pressure are lacking.
There is a need for an atmospheric-pressure plasma source with an improved jet impingement configuration that enables more effective delivery of a plasma-generated reactive species to the surface of an object. There is also a need for a low-temperature, atmospheric-pressure plasma source that generates an uncontaminated flux of reactive gas that can be delivered to a surface in an efficient manner so that the plasma-generated species can be used to rapidly treat both flat and 3-dimensional substrates of any size or shape. Furthermore, inefficient mass transport of plasma-excited reactive gases to a substrate surface is a hindrance to the further development of atmospheric-pressure plasma technology.